Join Susan as she obsesses about cosmetic chemistry and other things (some possibly related to monkeys). Often strange, occasionally useful, and always worth a stop as a point of interest on your journey through the Intertron.

Tuesday, February 28, 2012

Question: Does glycerin draw water from your skin when the humidity is low?

I'm still on a quest to figure out the answer to this question...so let's review information I've found so far.

What is a humectant? "Humectancy or hygroscopy is the tendency of a substance to attract water from the surroundings by absorption and adsorption at defined conditions (temperature, humidity)." (page 26, this review).

Is glycerin a humectant? Yes. "Pure glycerol absorbs its own weight in water over 3 days." (page 23, this review)

Does glycerin remove water from your skin in low humidity environments? Let's review what I've found so far...

From SW Gloves News: "Glycerin is also a humectant, which means it attracts moisture to your skin. However, if the air is less than 65% humidity, glycerin will instead draw moisture out of the lower layers of your skin. The moisture is held on the surface of your skin, effectively drying it from the inside out. This results in the upper layers of skin feeling soft and conditioned, while in reality the newly formed inner layers are drying out." (They are referencing this site Natural Health Information Centre, which adds, "It doesn't take a rocket scientist to work out that if you keep drying out the skin from the inside out, it will get progressively worse!") There is no citation from the Natural Health Information Centre about where they found this information.

From Dermatology Times, we get this quote,"While glycerin is a highly effective moisturizing ingredient in low-humidity climates (such as those found in Arizona, Nevada and New Mexico), it can leave skin with a sticky feel in humid environments (such as those found in Florida, Louisiana and Mississippi.) This is because glycerin can also attract water from the air." I'm inclined to go with the words of Zoe Diana Draelos, M.D., over SW Gloves News and Natural Health Information Centre, but let's not stop here!

From The Biology of Women, from a chapter entitled, Cosmetics: The Multibillion Dollar Put-On, we see this quote, "Advertisements imply that such agents have the ability to draw water out out of the air and bring it to the skin surface. It is very unlikely that humectants actually transfer water out of the atmosphere to the skin. They are, in fact, nondirectional. If humectants were used alone or in high enough proportions they would be just as likely to pull water out of the skin. The only way that a high concentration of glycerine (or glycerol, which is the same substance) can be useful for dry skin is under conditions of very high atmospheric humidity of 90% or more. Of course, if the humidity is very high, dry skin would not be a problem."

I had wanted to present these quotes without comment, but I don't think I can in this case as most of the preceding paragraph is simply inaccurate. I want to highlight the sentence "If humectants were used alone...they would be just as likely to pull water out of the skin" because this is the crux of the entire argument about glycerin drawing water out of your skin when it isn't humid. Remember that part - if humectants were used alone - and ignore most of the rest of the paragraph. I just think things need to be put into context, and to take that one phrase out of this wildly inaccurate paragraph would be cherry picking information to make my case, and I hate that!

"They [humectants] are able to attract water from the atmosphere (if the atmospheric humidity is greater than 80%) and from the underlying dermis. Although humectants may draw water from the environment to help hydrate the skin, in low humidity conditions, they may take water from the deeper epidermis and dermis, resulting in increased skin dryness. For this reason, they work better with occlusives." (Cosmetic Dermatology: Principles and Practice by Leslie Baumann, page 94.)

"Creating an occlusive barrier is more effective in combination with a humectant to draw water to the stratum corneum. Using a humectant alone will draw water to the outer surface of the skin, which will be lost to the environment with an impaired [skin] barrier. However, moisturizers without humectants may also make the skin more susceptible to irritation." (Lippincott's Primary Care Dermatology, page 31).

From this study, "In every experiment, the temperature was held constant at 25˚C, and the starting relative humidity (RH) was set to 0% RH. The RH was programmed to step in 10% increments ending at 90% RH. The RH was incremented to the next level only when the mass change was less than 0.005%/min. NaPCA [sodium PCA] was a more effective humectant at high relative humidities (above 60% RH); while glycerin performed better at humidities below 40% RH. Washing the skin with sodium lauryl sulfate (SLS) reduced the skin’s ability to absorb water more so than washing with Tween 80, a milder surfactant. Vaseline petroleum jelly enhanced the water-retention properties of untreated skin." From page 39, "Glycerin outperforms NaPCA at humidities below 40%." Page 40, "Clearly, Table 3 shows that the skin samples treated with Vaseline experienced a lower rate of water loss." Page 40, "Of the moisturizers that were examined in this study, glycerin was one of the most efficient at absorbing water at low relative humidities." Page 41, "Because occlusion prevents water evaporation, occluded skin has a higher water content compared with unoccluded skin. Occlusion can be achieved mechanically or chemically, by applying a hydrophobic material or film."

"...glycerol does not act as humectant at such a low humidity..." (This review, page 27, near the bottom)

In this study, the abstract notes that "In vitro studies have shown glycerin to prevent crystallization of stratum corneum model lipid mixture at low room humidity."

What can we conclude from these studies and writings? Humectants draw water out of the atmosphere to skin, but when it is less humid, humectants may draw water from under the skin to outside the skin (remember the idea of osmosis!). Not just glycerin, but any humectant, including honeyquat, sodium lactate, sodium PCA, tamarind seed extract, and so on: Any humectant has this potential. If we include an occlusive ingredient with the humectant, this should stop the water from evaporating and will help moisturize our skin.

A couple of questions...

Why is the idea of drawing water out of your skin only about glycerin and never about the other humectants?

How much glycerin do I need to use for this effect to make my skin really dry?

Why do the authors of scientific studies of glycerin - and other humectants - never mention the humidity in their part of the world? Wouldn't this have a huge impact on the results of the study?

At what humidity level does it draw water from your skin? If it is a well known fact that glycerin draws water from your skin, wouldn't the humidity level at which this starts to happen also be a well known fact? There should be some kind of chart or information showing that at this humidity and below, glycerin is a dessicant and above this humidity and higher, glycerin is a humectant. I can't find anything like that!

There is evidence demonstrating that although glycerin might not be a good humectant at low humidity levels, it offers other benefits - for instance, helping to keep our stratum corneum lipids from crystallizing - more about this in a near future post - which makes it very valuable, indeed! And there's some evidence a'brewing that use of a humectant can increase mildness in our products. (I will be writing about this in the very near future!)

Would I say that glycerin draws water from your skin when the humidity is low based on what I've read? No. I think there is that potential from a chemistry perspective, but I think there's strong evidence that using an occlusive ingredient with glycerin - or any humectant - will prevent this from happening in real life. I think there's way too much evidence showing that glycerin performs amazingly well as a moisturizer that can offer a ton of benefits beyond drawing water to my skin to not use it. I do think that most of the information about glycerin drawing water from our skin comes from popular media or websites, and I don't think this claim is substantiated.

Would I use glycerin in my products at 20% humidity? At 10%? At 5%? Yes, and I have. I would not hesitate to suggest using glycerin in products for low humidity environments. I think there are other humectants that might perform better in those environments, but it sounds like none of the humectants we can access - see list above! - will work well in a really dry environment. You'll have to get your moisturization from somewhere else!

14 comments:

I wonder if they have done any actual studies in a low humidity environment. When I used to live in the High Desert of Az, I never had a problem using glycerin or any other humectant. Of course, that isn't scientific. I have had super dry skin before, looked like alligator scales, but recovered after I discovered that my job around cardboard was sucking the moisture from my arms and therefore I needed to cover up and remember to use lotion regularly.

Here in New Mexico regular lotions (glycerin-based) never seem to work all that great. But if I wet my hands and then put shea butter on them (no other oils in it) my hands stay moisturized for a day. I am guessing this is the occlusive effect you were talking about.

Now, if I were to create a lotion using glycerin as my humectant since it's easy to get and I already have it, which occlusive ingredients would retain moisture best? A butter, like shea or cocoa butter? Or mineral oil? Or something else?

@Zenobia: Please don't use mineral oil. It is composed only of C and H. It has no affinity or benefits for human tissue - in fact quite the opposite. Manufacturers of cheap products use it because it's dirt cheap. It isn't biodegradable.

@Zenobiah: I forgot to reply to your question. Shea butter is great for skin, you'll notice it has become very popular in skin care products. It contains retinol, and continued use will, you'll find, dramatically fade photo-age spots. But take the trouble to obtain Class A unrefined. The refined stuff has lost many excellent qualities. Kpnangan butter is also great stuff, leaves a very silky feeling, but it is not easy to get hold of. Other good emollients would be almond oil and avocado oil.

Thanks, Clive. I always use unrefined shea butter (it's even organic). Mineral oil is an occlusive oil and works for certain applications. It's not my favorite oil, but it does have its uses in skincare. I am just looking for something that will slow down the evaporation we experience here in the desert.

I live in Houston, Tx, and humidity here is very high, therefore, i should keep adding 3or 4% glycerin in my leave in conditioner, is it right? Another question is, could one add more glycerin and what could possibly happen to the hair if one adds too much glycerin?Thanks,Rosi

If you want to know more about shea butter, check out this post I've written on the topic. It's a great butter for softening and moisturizing. I can't use anything but unrefined due to the smell! It's just too earthy for my tastes!

Clive: Where did you read that shea butter contains retinol? Can you send it to me? I'd love to know more about this!

I don't think there's anything wrong with using mineral oil, it's just that there are other oils out there that offer much more to our skin like polyphenols, phytosterols, and lovely fatty acids. I've used mineral oil in my moisturizer as I wanted something non-occlusive that wouldn't make me break out, and it worked very well.

I'm researching hair and humectants right now...more on this soon. As for if there's too much in a conditioner, trust me, you'll know. I had a 5% glycerin conditioner and it felt awful, very sticky. I wouldn't go over 3% in a rinse off and 1% to 2% in a leave in!

The other thing that occurs to me when I read this is that the body is a replenishing system. OK, so glycerine pulls water from the lower layers of skin to the upper layers, depending on outside humidity blah blah. And if you don't have an occlusive ingredient, that water then evaporates off. But, its not like those lower layers have no ability to refurbish their water content. The body is a finely tuned system. Those cells are going to put out a call (or simple osmosis will take over) and water that you have consumed will be replenished into that lower layer of cells. So unless you are severely dehydrated, I'd rather have the upper layer of my skin be less dry than the lower layers be moist and the upper layers be dry and itchy. The lotion I make contains a lot of glycerine. Its the 3rd ingredient by weight. And I used it for 10 years in 35% humidity Colorado. And it worked like a charm. Of course, I had occlusive ingredients in there too - cocoa butter for one. Jennifer

What if you wanted to purposely draw water/fluid from the lower layers of skin. In a situation where the skin is so bloated with water- not in the muscle but in the skin that diuretics can only do so much and are taxing to the kidneys and compression has become ineffective. Could this be used as another prong of attack?

My name is Tom. I am still somewhat confused about humectants. From my understanding, I thought humectants only pulled water from the air. Humidty would come in to play because if there is no humidty, there is no water in the air to pull from. Could you please answer the following question because I still do not know the answer even after reading your notes. Can a humectant make your skin drier even if there is no water in the air (humidty)? If there is no water in the air, would not the humectant just be deemed neutral? Neutral meaning that it is not going to have a positive effect (pulling water) nor a negative effect (making skin drier). It will basically just do nothing, it will add no real benefit to the product. (I know humectants can have other beneficial properties besides pulling water, but in this case, I am strictly talking about pulling water and making the skin drier).

Could you take a look at the following site. Scroll down to the humectant section, it says that humectants do not pull water from the air at all.

Thank you for this post.I live in north Europe, so in winter time, when it gets -20C degrees, the weather is really dry. This is the only season I have to use moisturisers. And they only show short-time effects, the skin is very dry most of the time. Im even thinking it gets worse. Any tips from you?

Hi Gie. I really need more information than what you're offering. Are you in a high humidity or low humidity place? Are you looking for something to create a barrier on your skin while you're in the cold, wind, or snow or do you want something afterwards? Can you add more information? Thanks!

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